JP6350686B2 - Austenitic stainless steel - Google Patents

Description

  The present invention relates to austenitic stainless steel.

In Japan, since the 1990s, high-temperature and high-pressure boilers have progressed, and super super critical power (USC) boilers with steam temperatures exceeding 600 ° C. have become mainstream.
On the other hand, Europe, including China, even in the world of the boiler, from the point of view of CO ₂ reduction of global environmental protection, high efficiency of USC boiler has been one after another construction.
As a material steel used for heat exchanger pipes and boiler pipes that generate high-temperature and high-pressure steam in a boiler, steel materials having high high-temperature strength are desired, and various steel materials have been developed in recent years.

For example, Patent Document 1 discloses 18Cr austenitic stainless steel having excellent high-temperature strength and excellent steam oxidation resistance.
Patent Document 2 discloses an austenitic stainless steel excellent in high temperature corrosion thermal fatigue crack resistance.
Patent Document 3 discloses a heat-resistant austenitic stainless steel excellent in high-temperature strength and resistance to repeated oxidation.

Patent Document 4 discloses an austenitic stainless steel having excellent toughness even after being exposed to a high temperature environment for a long time.
Patent Document 5 discloses a high-strength austenitic stainless steel having a creep rupture strength at 800 ° C. × 600 hours of 100 MPa or more.

  Patent Document 6 describes a method for securing high-temperature strength by utilizing solid solution strengthening and precipitation strengthening of nitride by adding a large amount of N (nitrogen) to compensate for the low strength of low carbon stainless steel (a large amount of N addition method). Is disclosed.

Japanese Patent No. 3632672 Japanese Patent No. 5029788 Japanese Patent No. 5143960 Japanese Patent No. 5547789 Japanese Patent No. 5670103 Japanese Patent No. 3388998

  In general, in the design of the composition of the material steel used for heat exchanger tubes used in high temperature regions and boiler piping used in high temperature regions, high temperature strength (for example, creep strength), high temperature corrosion resistance, steam oxidation resistance, heat fatigue resistance On the other hand, corrosion resistance (for example, stress corrosion cracking resistance in water) in a temperature range from room temperature to around 350 ° C. is not emphasized. The reason is that the corrosion resistance in the temperature range from room temperature to around 350 ° C. has conventionally been dealt with by the construction technique or the operation management technique.

However, in recent years, stress corrosion cracking has occurred in water at normal temperature and low temperature (about 350 ° C or less) due to non-uniform metal structure or precipitation of non-uniform carbides in heat-treated parts such as welds and bending parts. It has become a big problem.
For example, when a boiler water pressure test is performed or when the operation of the boiler is stopped, water stays in the heat exchanger tube for a long time, and at this time, stress corrosion cracking may occur remarkably.

  The stress corrosion cracking of stainless steel occurs due to the selective precipitation of Cr-based carbides or the formation of a low Cr concentration layer (Cr-deficient layer) in the vicinity of the crystal grain boundary, which tends to be selectively corroded.

As a method for preventing stress corrosion cracking of 18Cr austenitic stainless steel, conventionally, a method of reducing the amount of C and suppressing the formation of grain boundary Cr carbide (low carbonization method),
In order to suppress the formation of grain boundary Cr carbide, Nb and Ti, which have higher carbide forming ability than Cr, are added to form MC carbide, and C is fixed (stabilized heat treatment method). To suppress the formation of Cr-deficient layers and to suppress selective corrosion at grain boundaries (a large amount of Cr addition method),
Etc. are known.

However, both methods have problems.
In the low carbonization method, carbides effective for high temperature strength are not generated, and high temperature strength tends to decrease.
In the stabilization heat treatment method, the stabilization heat treatment must be performed at a temperature as low as about 950 ° C., and the high-temperature strength, particularly the creep strength, tends to be impaired.
In a large amount of Cr addition method, since a large amount of brittle phases such as sigma phase are generated, it is necessary to add a large amount of expensive Ni in order to stabilize the metal structure and maintain high temperature strength. Tend to rise.

The method described in Patent Document 6 (a large amount of N addition method) is a method devised as a method that replaces the above-described conventional method.
In order to compensate for the low strength of low carbon stainless steel, the large amount N addition method is a method of securing high temperature strength by utilizing solid solution strengthening and nitride precipitation strengthening by adding a large amount of N.

  However, in the method of Patent Document 6 (a method of adding a large amount of N), a large amount of nitride is generated and, on the contrary, a problem of stress corrosion cracking occurs, or sufficient high-temperature strength is obtained at a high temperature range of 700 ° C. or higher. It turned out that there was a problem that was not possible.

Due to the circumstances described above, 18Cr-based austenitic stainless steel has excellent high-temperature strength and stress corrosion cracking resistance regardless of the conventional low carbonization method, stabilized heat treatment method, large amount Cr addition method, and large amount N addition method. Is required to ensure.
An object of the present invention is to provide an 18Cr-based austenitic stainless steel that has excellent high-temperature strength and stress corrosion cracking resistance.

  Means for solving the above problems include the following aspects.

<1> Ingredient composition is mass%,
C: 0.05 to 0.13%,
Si: 0.10 to 1.00%,
Mn: 0.10 to 3.00%,
P: 0.040% or less,
S: 0.020% or less,
Cr: 17.00-19.00%,
Ni: 12.00 to 15.00%,
Cu: 2.00 to 4.00%,
Mo: 0.01-2.00%,
W: 2.00 to 5.00%
2Mo + W: 2.50 to 5.00%,
V: 0.01-0.40%,
Ti: 0.05 to 0.50%,
Nb: 0.15 to 0.70%,
Al: 0.001 to 0.040%,
B: 0.0010 to 0.0100%
N: 0.0010 to 0.0100%,
Nd: 0.001 to 0.20%
Zr: 0.002% or less,
Bi: 0.001% or less,
Sn: 0.010% or less,
Sb: 0.010% or less,
Pb: 0.001% or less,
As: 0.001% or less,
Zr + Bi + Sn + Sb + Pb + As: 0.020% or less,
O: 0.0090% or less,
Co: 0.80% or less,
Ca: 0.20% or less,
Mg: 0.20% or less,
One or more of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re: 0.20% or less in total, and
The balance: Fe and impurities,
An austenitic stainless steel having an effective M amount Meff defined by the following formula (1) of 0.002 to 0.250%.

Effective M amount Meff = Nd + 13 · (B-11 · N / 14) −1.6 · Zr (1)
(In the formula (1), each element symbol indicates the content (% by mass) of each element.)

<2> The component composition is one by weight in terms of Co: 0.01 to 0.80%, Ca: 0.0001 to 0.20%, and Mg: 0.0005 to 0.20%. The austenitic stainless steel as described in <1> containing 2 or more types.
<3> The component composition contains 0.001 to 0.20% in total of one or more of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re in mass%. > Or <2> The austenitic stainless steel according to <2>.
<4> The austenitic stainless steel according to any one of <1> to <3>, wherein the creep rupture strength at 700 ° C. and 10,000 hours is 140 MPa or more.

  According to the present invention, an austenitic stainless steel which is 18Cr austenitic stainless steel and has excellent high temperature strength and stress corrosion cracking resistance is provided.

Hereinafter, embodiments of the present invention will be described.
In the present specification, a numerical range represented using “to” means a range including numerical values described before and after “to” as a lower limit value and an upper limit value.
In the present specification, “%” indicating the element content and “%” indicating the value of the effective M amount Meff mean “mass%”.
In the present specification, the content of C (carbon) may be expressed as “C amount”. The content of other elements may be expressed in the same manner.

  The austenitic stainless steel of this embodiment (hereinafter, also referred to as “steel of this embodiment”) has a component composition of mass%, C: 0.05 to 0.13%, Si: 0.10 to 1.00. %, Mn: 0.10 to 3.00%, P: 0.040% or less, S: 0.020% or less, Cr: 17.00 to 19.00%, Ni: 12.00 to 15.00% Cu: 2.00 to 4.00%, Mo: 0.01 to 2.00%, W: 2.00 to 5.00%, 2Mo + W: 2.50 to 5.00%, V: 0.01 To 0.40%, Ti: 0.05 to 0.50%, Nb: 0.15 to 0.70%, Al: 0.001 to 0.040%, B: 0.0010 to 0.0100%, N: 0.0010 to 0.0100%, Nd: 0.001 to 0.20%, Zr: 0.002% or less, Bi: 0.001% or less, n: 0.010% or less, Sb: 0.010% or less, Pb: 0.001% or less, As: 0.001% or less, Zr + Bi + Sn + Sb + Pb + As: 0.020% or less, O: 0.0090% or less, Co: 0.80% or less, Ca: 0.20% or less, Mg: 0.20% or less, one or more of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re: It consists of 20% or less and the balance: Fe and impurities, and the effective M amount Meff defined by the following formula (1) is 0.0001 to 0.25%.

Effective M amount Meff = Nd + 13 · (B-11 · N / 14) −1.6 · Zr (1)
(In the formula (1), each element symbol indicates the content (% by mass) of each element.)

The chemical composition of the steel of this embodiment contains Cr: 17.00-19.00%.
That is, the steel of this embodiment belongs to 18Cr austenitic stainless steel.
As described above, 18Cr-based austenitic stainless steel has excellent high-temperature strength and stress corrosion cracking resistance regardless of the conventional low carbonization method, stabilized heat treatment method, large amount Cr addition method, and large amount N addition method. It is required to ensure.

According to the steel of this embodiment, excellent high-temperature strength and stress corrosion cracking resistance are ensured regardless of the conventional low carbonization method, stabilization heat treatment method, large amount Cr addition method, and large amount N addition method. The
The reason why this effect is achieved by the steel of this embodiment is presumed as follows. However, the present invention is not limited by the following assumptions.

In the steel of this embodiment, Nd and B are added together at the above contents, and further, the effective M amount Meff is adjusted to be in the above range, whereby grain boundary cleaning and strength improvement are achieved. .
Furthermore, in the steel of the present embodiment, by limiting the contents of impurities Zr, Bi, Sn, Sb, Pb, and As (hereinafter also referred to as “impurity 6 elements”) to the above range, High purity is achieved.
The above-described grain boundary cleaning, strength improvement, and high purity make it possible to achieve excellent high-temperature strength and stress corrosion cracking resistance regardless of any of the low carbonization method, stabilization heat treatment method, and large amount of Cr addition method. Is considered to be secured.

Furthermore, in the steel of this embodiment, N (nitrogen) is reduced as much as possible (specifically, 0.0100% or less), and by adding the above amount of W, fine carbide precipitation and fine and stable It is considered that precipitation strengthening by the precipitation of the Laves phase becomes possible.
As a result, it is considered that excellent high-temperature strength is secured in the 18Cr austenitic stainless steel without depending on a large amount of N addition method (for example, see Patent Document 6).
This finding is a novel finding contrary to conventional common sense.

In general, carbides and Laves phases are preferentially precipitated around nitrides and nitrides at grain boundaries, thereby impairing high temperature strength and corrosion resistance. That is, if nitride is present, precipitation of fine carbide and precipitation of a fine and stable Laves phase are difficult, and the high-temperature strength is not improved. In particular, when coarse Zr nitride is present, the precipitation of fine carbides and the precipitation of fine and stable Laves phases become more difficult, so it is necessary to reduce N and Zr as much as possible.
However, a very small amount of N forms fine carbide precipitation nuclei that contribute to the improvement of high-temperature strength. For this reason, in the steel of this embodiment, N is managed as a useful element, not an impurity element, in an extremely low amount region (specifically, 0.0010 to 0.0100%).
In the steel of this embodiment, by setting the N amount to 0.0010 to 0.0100%, both precipitation strengthening by fine carbides and precipitation strengthening of a fine and stable Laves phase are effectively achieved. In a temperature range of 700 ° C. or higher, high-temperature strength can be secured and the metal structure can be stabilized.
That is, in the steel of this embodiment, high strength can be achieved without depending on precipitation strengthening of nitrides, and stabilization of the metal structure can be achieved without the generation of an embrittlement phase or the like. This method is a method not found in the prior art.

  Hereinafter, first, the chemical composition of the steel of the present embodiment and a preferred aspect thereof will be described, and then the effective M amount Meff (formula (1)) and the like will be described.

C: 0.05 to 0.13%
C is an element indispensable for the formation of carbides and stabilization of the austenite structure, and further improvement of high temperature strength and stabilization of the metal structure at high temperature.
The steel of this embodiment can prevent stress corrosion cracking without using N addition strengthening and without reducing C.
However, if the C content is less than 0.05%, it is difficult to improve the high-temperature creep strength and stabilize the metal structure at a high temperature, so the C content is set to 0.05% or more. The amount of C is preferably 0.06% or more.

  On the other hand, if the C content exceeds 0.13%, coarse Cr carbide precipitates at the grain boundaries, causing stress corrosion cracking or weld cracking, and lowering toughness. For this reason, the amount of C is made into 0.13% or less. The amount of C is preferably 0.12% or less.

Si: 0.10 to 1.00%
Si is an element that functions as a deoxidizer during steelmaking and prevents steam oxidation at high temperatures. However, if the Si amount is less than 0.10%, the effect of addition cannot be obtained sufficiently, so the Si amount is set to 0.10% or more. The amount of Si is preferably 0.20% or more.

  On the other hand, if the Si content exceeds 1.00%, the workability deteriorates and an embrittlement phase such as a sigma phase precipitates at a high temperature, so the Si content is set to 1.00% or less. The amount of Si is preferably 0.80% or less.

Mn: 0.10 to 3.00%
Mn is an element that contributes to the stabilization of the metal structure at high temperatures, while forming impurity elements S and MnS to detoxify S and contribute to improving hot workability.
However, if the Mn content is less than 0.10%, the effect of addition cannot be obtained sufficiently, so the Mn content is 0.10% or more. The amount of Mn is preferably 0.20% or more.
On the other hand, if the amount of Mn exceeds 3.00%, workability and weldability deteriorate, so the amount of Mn is set to 3.00% or less. The amount of Mn is preferably 2.60% or less.

P: 0.040% or less P is an impurity element and is an element that hinders workability and weldability.
When the amount of P exceeds 0.040%, workability and weldability are significantly reduced. For this reason, the amount of P is made into 0.040% or less. The amount of P is preferably 0.030% or less, and more preferably 0.020% or less.

The smaller the amount of P, the better. Therefore, the amount of P may be 0%.
However, P may be inevitably mixed from a steel raw material (raw ore, scrap, etc.), and if the amount of P is reduced to less than 0.001%, the manufacturing cost increases significantly. For this reason, from the viewpoint of manufacturing cost, the P amount may be 0.001% or more.

S: 0.020% or less S is an impurity element, and is an element that inhibits workability, weldability, and stress corrosion cracking resistance.
When the amount of S exceeds 0.020%, workability, weldability, and stress corrosion cracking resistance are significantly reduced. For this reason, the amount of S is made into 0.020% or less.
In order to improve the hot water flow during welding, S may be added, but in that case, 0.020% or less is also added. The amount of S is preferably 0.010% or less.

Since S is preferably as small as possible, the S amount may be 0%.
However, S may be inevitably mixed from a steel raw material (raw ore, scrap, etc.), and if the amount of S is reduced to less than 0.001%, the manufacturing cost increases significantly. For this reason, 0.001% or more may be sufficient as S amount from a viewpoint of manufacturing cost.

Cr: 17.00 to 19.00%
Cr is an element that contributes to the improvement of oxidation resistance, steam oxidation resistance, stress corrosion cracking resistance, and strength and metal structure stabilization due to Cr carbide, as the main elements of 18Cr austenitic stainless steel.
If the Cr content is less than 17.00%, the effect of addition cannot be obtained sufficiently. For this reason, the Cr content is set to 17.00% or more. The amount of Cr is preferably 17.30% or more, and more preferably 17.50% or more.

  On the other hand, if the Cr content exceeds 19.00%, a large amount of Ni is required to maintain the stability of the austenite structure, and an embrittled phase is generated, resulting in a decrease in high-temperature strength and toughness. For this reason, the Cr amount is set to 19.00% or less. The amount of Cr is preferably 18.80% or less, and more preferably 18.60% or less.

Ni: 12.00 to 15.00%
Ni is an austenite-forming element, and is an element that contributes to improvement of high-temperature strength and workability and stabilization of the metal structure at high temperature as a main element of 18Cr-based austenitic stainless steel.

  When the amount of Ni is less than 12.00%, the effect of addition is not sufficiently obtained, and the amount of Ni, such as Cr, W, and Mo, is lacking in balance with the amount of ferrite-forming elements, and the embrittlement phase (sigma phase, etc.) Promote the generation of For this reason, the amount of Ni is made 12.00% or more. The amount of Ni is preferably 12.50% or more.

  On the other hand, if the Ni content exceeds 15.00%, the high-temperature strength and economy are lowered, so the content is made 15.00% or less. The amount of Ni is preferably 14.90% or less, more preferably 14.80% or less, and still more preferably 14.50% or less.

Cu: 2.00 to 4.00%
Cu is an element that is fine and precipitates as a Cu phase that is stable at a high temperature and contributes to an increase in high-temperature strength.
If the amount of Cu is less than 2.00%, the effect of addition cannot be obtained sufficiently, so the amount of Cu is made 2.00% or more. The amount of Cu is preferably 2.20% or more, more preferably 2.50% or more.

  On the other hand, when the amount of Cu exceeds 4.00%, workability, creep ductility, and strength decrease. Therefore, the Cu amount is 4.00% or less. The amount of Cu is preferably 3.90% or less, more preferably 3.80% or less, and still more preferably 3.50% or less.

Mo: 0.01 to 2.00%
Mo is an element indispensable for improving corrosion resistance, high-temperature strength, and stress corrosion cracking resistance. Mo is an element that contributes to the formation of Laves phases and carbides that are stable at high temperatures for a long time due to the synergistic effect of the combined addition with W.

  If the amount of Mo is less than 0.01%, the effect of addition cannot be sufficiently obtained, so Mo is made 0.01% or more. The amount of Mo is preferably 0.02% or more.

  On the other hand, if the Mo content exceeds 2.00%, a large amount of embrittlement phase is generated, and the workability, high-temperature strength, and toughness are reduced, so Mo is made 2.00% or less. The amount of Mo is preferably 1.80% or less, more preferably 1.50% or less, and still more preferably 1.30% or less.

W: 2.00 to 5.00%
W is an element indispensable for improving corrosion resistance, high temperature strength, and stress corrosion cracking resistance. In addition, it is an element that contributes to the precipitation of Laves phase and carbides that are stable at high temperatures for a long time due to a synergistic effect by combined addition with Mo. Further, W is an element that contributes to stable long-term strength maintenance at a high temperature because diffusion at a higher temperature is slower than that of Mo.

  If the amount of W is less than 2.00%, the effect of addition cannot be obtained sufficiently, so the amount of W is made 2.00% or more. The amount of W is preferably 2.10% or more.

  On the other hand, if the amount of W exceeds 5.00%, a large amount of embrittlement phase is generated, and the workability and strength are lowered. Therefore, the amount of W is made 5.00% or less. The amount of W is preferably 4.90% or less, more preferably 4.80% or less, and still more preferably 4.70% or less.

2Mo + W: 2.50 to 5.00%
The combined addition of Mo and W contributes to the improvement of high temperature strength, stress corrosion cracking resistance, and high temperature corrosion resistance. If 2Mo + W (where Mo represents the amount of Mo and W represents the amount of W. The same shall apply hereinafter) is less than 2.50%, the synergistic effect due to the composite addition cannot be sufficiently obtained. For this reason, 2Mo + W is set to 2.50% or more. 2Mo + W is preferably 2.60% or more, more preferably 2.80% or more, and further preferably 3.00% or more.

  On the other hand, when 2Mo + W exceeds 5.00%, the strength and toughness are lowered, and the stability of the metal structure at high temperatures is lowered. For this reason, 2Mo + W is 5.00% or less. 2Mo + W is preferably 4.90% or less.

V: 0.01-0.40%
V is an element that forms fine carbides together with Ti and Nb and contributes to the improvement of high-temperature strength. If the amount of V is less than 0.01%, the effect of addition cannot be obtained sufficiently, so the amount of V is set to 0.01% or more. The amount of V is preferably 0.02% or more.

  On the other hand, if the V content exceeds 0.40%, strength and stress corrosion cracking resistance deteriorate, so the V content is set to 0.40% or less. The amount of V is preferably 0.38% or less.

Ti: 0.05 to 0.50%
Ti, together with V and Nb, forms fine carbides and contributes to the improvement of high-temperature strength, and also fixes C and suppresses precipitation of Cr carbides at grain boundaries, thereby improving stress corrosion cracking resistance. It is a contributing element.

In conventional N-added austenitic stainless steel, nitride precipitates in a lump and the effect of N addition does not appear effectively, and coarse Cr carbide precipitates at the grain boundaries, reducing the stress corrosion cracking resistance. .
The present inventors have demonstrated that the useful effects of fine Ti carbides can be achieved by controlling the N content at an extremely low level in 18Cr austenitic stainless steel. Specifically, fine Ti carbides can be produced. It has been found that a fine Laves phase precipitates as a nucleus, and as a result, the high-temperature strength of the steel is greatly improved.

  If the amount of Ti is less than 0.05%, the effect of addition cannot be obtained sufficiently, so the amount of Ti is set to 0.05% or more. Combined addition with Nb and V is preferred, and the Ti content is preferably 0.10% or more.

  On the other hand, if the Ti content exceeds 0.50%, massive precipitates are deposited and the strength, toughness, and stress corrosion cracking resistance are deteriorated, so the Ti content is 0.50% or less. The amount of Ti is preferably 0.45% or less.

Nb: 0.15 to 0.70%
Nb, together with V and Ti, forms fine carbides and contributes to the improvement of high-temperature strength, and also fixes C and suppresses the precipitation of Cr carbides at the grain boundaries, thereby improving the stress corrosion cracking resistance. It is a contributing element.
Nb, like Ti, is an element that contributes to the improvement of high-temperature strength due to the precipitation of a fine Laves phase.

If the Nb content is less than 0.15%, the effect of addition cannot be obtained sufficiently, so the Nb content is 0.15% or more. The Nb amount is preferably 0.20% or more.
On the other hand, if the amount of Nb exceeds 0.70%, massive precipitates are deposited, and the strength, toughness, and stress corrosion cracking resistance are reduced. Therefore, the amount of Nb is set to 0.70% or less. The Nb amount is preferably 0.60% or less.

Al: 0.001 to 0.040%
Al is an element that functions as a deoxidizing element during steelmaking and cleans steel.
If the Al amount is less than 0.001%, the steel cannot be sufficiently cleaned, so the Al amount is set to 0.001% or more. The amount of Al is preferably 0.002% or more.

  On the other hand, when the Al content exceeds 0.040%, a large amount of non-metallic inclusions are generated, and the stress corrosion cracking resistance, high-temperature strength, workability, toughness, and stability of the metal structure at high temperatures are reduced. Therefore, the Al content is set to 0.040% or less. The amount of Al is preferably 0.034% or less.

B: 0.0010 to 0.0100%
B is an element for achieving excellent high-temperature strength and stress corrosion cracking resistance by complex addition with Nd, which is important in the steel of this embodiment, and is an indispensable element. B segregates at the grain boundaries and contributes not only to the improvement of the high temperature strength but also to the formation of carbides, the refinement of the Laves phase, and the stabilization of the metal structure, which are effective for the improvement of the high temperature strength. It is an element.

  Further, B is an element that renders N (0.0010 to 0.0100% in the steel of the present embodiment) harmless as BN and contributes to improvement in high temperature strength and stress corrosion resistance.

If the amount of B is less than 0.0010%, B that exists without being consumed as a nitride, that is, B that contributes to improvement in high-temperature strength and stress corrosion resistance cannot be secured. For this reason, if the amount of B is less than 0.0010%, a synergistic effect (this point will be described later) due to the combined addition with Nd (and securing of effective M amount) cannot be obtained, so high temperature strength and stress resistance Corrosion cracking is not improved. Therefore, the B amount is 0.0010% or more.
The amount of B is preferably 0.0015% or more.

  On the other hand, if the amount of B exceeds 0.0100%, a boron compound is generated, and the workability, weldability, and high-temperature strength decrease, so the amount of B is set to 0.0100% or less. The amount of B is preferably 0.0080% or less, and more preferably 0.0060% or less.

N: 0.0010 to 0.0100%
N (nitrogen) is a useful element for improving the high-temperature strength by solid solution strengthening and precipitation strengthening of nitride in general 18Cr austenitic stainless steel. However, in the steel of this embodiment, N inhibits the stress corrosion cracking resistance, so N is not actively added.
However, since a small amount of N generates precipitation nuclei of fine precipitates effective for improving high-temperature strength, in the steel of this embodiment, a small amount of fine precipitates generating nuclei of fine precipitates effective for improving high-temperature strength is produced. Allow N in the range.

  That is, the basic idea of the steel of the present embodiment is different from the prior art in that N is not actively added and N is allowed in a very small range.

  If the amount of N is less than 0.0010%, it is difficult to form precipitation nuclei of fine precipitates effective for improving the high temperature strength, so the amount of N is set to 0.0010% or more. The amount of N is preferably 0.0020% or more, more preferably 0.0030% or more.

  On the other hand, if the N content exceeds 0.0100%, nitrides are generated and the high-temperature strength and stress corrosion cracking resistance deteriorate, so the N content is 0.0100% or less. The amount of N is preferably 0.0090% or less, more preferably 0.0080% or less, and still more preferably 0.0070% or less.

Nd: 0.001 to 0.20%
Nd is an element that remarkably improves high-temperature strength and stress corrosion cracking resistance due to a synergistic effect (described later) by the combined addition with B.
As described above, in the steel of this embodiment, the carbide and Laves phase effective for improving the high-temperature strength are refined, and long-term stability is ensured. Strengthen grain boundaries to improve stress corrosion cracking resistance.

  However, Nd has an extremely strong bonding force with N, O, and S, and even when added as metal Nd, it is deposited and consumed as a harmful precipitate, and the effect of addition is hardly exhibited. For this reason, in order to sufficiently obtain the Nd addition effect, it is necessary to reduce the N amount, the O amount, and the S amount as much as possible.

  If the amount of Nd is less than 0.001%, even if the amount of N, the amount of O, and the amount of S are reduced, the effect of adding Nd cannot be sufficiently obtained. Therefore, the Nd amount is 0.001% or more. The Nd amount is preferably 0.002% or more, more preferably 0.005% or more.

  On the other hand, when the amount of Nd exceeds 0.20%, the effect of addition is saturated and oxide inclusions are generated, and the strength, workability, and economy are lowered. Therefore, the Nd content is 0.20% or less. The amount of Nd is preferably 0.18% or less, more preferably 0.15% or less, and still more preferably 0.10% or less.

  The range of the Nd amount is preferably 0.002 to 0.15%, and more preferably 0.005 to 0.10%, in that the effective M amount Meff is more easily secured.

  In the steel of this embodiment, in order to ensure the excellent characteristics of the steel of this embodiment, Zr, Bi, Sn, Sb, Pb, As, and O are treated as impurity elements, and the amounts of these elements are limited.

  Usually, scraps such as alloy steel are mainly used as a raw material for stainless steel, but this scrap contains Zr, Bi, Sn, Sb, Pb, and As (impurity 6 elements), though in a small amount. ing. These six impurities are inevitably mixed in stainless steel (product).

  In addition, if the melting equipment is contaminated with other alloys in the manufacturing process of stainless steel, 6 elements of impurities are mixed into the stainless steel (product) from the melting equipment, and O (oxygen) Inevitably remains in stainless steel.

  In the steel of this embodiment, in order to ensure excellent high-temperature strength and stress corrosion cracking resistance, it is necessary to reduce Zr, Bi, Sn, Sb, Pb, As, and O as much as possible to obtain high-purity steel. .

Zr: 0.002% or less Zr is not normally mixed, but is mixed from scraps and / or melting equipment contaminated by the production of other alloys to form oxides and nitrides. The nitride functions as a nucleus from which a precipitate such as a Laves phase is deposited.
However, when a massive precipitate is deposited with nitride as a nucleus, high-temperature strength and stress corrosion cracking resistance are hindered.

  Thus, Zr is an element harmful to high temperature strength and stress corrosion cracking resistance. For this reason, in the relational expression (Equation (1)) of the effective M amount introduced in order to ensure excellent high-temperature strength and stress corrosion cracking resistance, “−1.6 · The term “Zr” was provided.

Since Zr is preferably as small as possible, the upper limit of the amount of Zr is 0.002% close to the analysis limit (0.001%). The amount of Zr is preferably 0.001% or less.
The amount of Zr may be 0%. However, Zr is inevitably mixed in by about 0.0001%. For this reason, from the viewpoint of manufacturing cost, the amount of Zr may be 0.0001% or more.

Bi: 0.001% or less Bi is not normally mixed, but is mixed from scraps and / or melting equipment contaminated by the production of other alloys, and inhibits high-temperature strength and stress corrosion cracking resistance. It is an element.
Since the Bi amount needs to be reduced as much as possible, the upper limit of the Bi amount is set to 0.001% of the analysis limit.
The amount of Bi may be 0%. However, Bi is inevitably mixed in by about 0.0001%. For this reason, from the viewpoint of manufacturing cost, the Bi amount may be 0.0001% or more.

Sn: 0.010% or less Sb: 0.010% or less Pb: 0.001% or less As: 0.001% or less Sn, Sb, Pb, and As are scraps and / or other alloys. It is an element that is easily mixed in from the melting equipment contaminated with, and difficult to remove during the refining process.
However, the amount of these elements must be reduced as much as possible.
Therefore, considering the raw material composition and the refining limit, the upper limits of the Sn amount and the Sb amount are each 0.010%. Each of the Sn amount and the Sb amount is preferably 0.005% or less.
Further, the upper limit of the Pb amount and the As amount is 0.001%, respectively. Pb and As are each preferably 0.0005% or less.

The Sn amount, Sb amount, Pb amount, and As amount may all be 0%.
However, these elements are inevitably mixed in by about 0.0001%. For this reason, from the viewpoint of production cost, the amount of any element may be 0.0001% or more.

Zr + Bi + Sn + Sb + Pb + As: 0.020% or less When the steel of the present invention inevitably contains Zr, Bi, Sn, Sb, Pb, and As (impurities 6 elements), it is excellent due to the synergistic effect of the combined addition of Nd and B. In order to secure high temperature strength and stress corrosion cracking resistance, not only the content of the six impurities is individually limited, but also the total content of the six impurities (Zr + Bi + Sn + Sb + Pb + As; where each element symbol is It is necessary to limit the content of each element.) To 0.020% or less to achieve higher purity.
In the steel of the present embodiment, the total content of the six impurities is 0.020% or less.
The total content of the six impurity elements is preferably 0.015% or less, more preferably 0.010% or less.
On the other hand, it is preferable that the total content of the six impurities is as small as possible in terms of ensuring excellent high-temperature strength and stress corrosion cracking resistance. For this reason, the lower limit of the total content of the six impurities is 0%.

O: 0.0090% or less O (oxygen) inevitably remaining after refining molten steel is an element that serves as an index of the amount of non-metallic inclusions.
If O exceeds 0.0090%, Nd oxide is generated and Nd is consumed, fine carbides or Laves phases are generated, and the effect of improving the high temperature strength and stress corrosion cracking resistance cannot be obtained. Therefore, the O amount is set to 0.0090% or less. The amount of O is preferably 0.0080% or less, more preferably 0.0070% or less, and still more preferably 0.0050% or less.

  The amount of O may be 0%. However, O may inevitably remain about 0.0001% after refining. For this reason, from the viewpoint of manufacturing cost, the O amount may be 0.0001% or more.

The composition of the steel of this embodiment is one or more of Co, Ca, and Mg, and / or one or two of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re. It may contain seeds or more.
These elements are all arbitrary elements. For this reason, the content of these elements may be 0%.

Co: 0.80% or less Co can be a source of contamination when manufacturing other steels. For this reason, the amount of Co is set to 0.80% or less. The amount of Co is preferably 0.60% or less.
Although the steel of this embodiment does not need to contain Co (that is, the amount of Co may be 0%), it contains Co from the viewpoint of further stabilizing the metal structure and further improving the high-temperature strength. May be.
When the steel of this embodiment contains Co, the amount of Co is preferably 0.01% or more, and more preferably 0.03% or more.

Ca: 0.20% or less Ca is an arbitrary element, and the amount of Ca may be 0%.
Ca is an element that can be added as a deoxidizing finish. Since the steel of this embodiment contains Nd, it is preferable to deoxidize with Ca in the refining process. In the case where the steel of the present embodiment contains Ca, the Ca content is preferably 0.0001% or more, more preferably 0.0010% or more from the viewpoint of obtaining a deoxidation effect more effectively.

  On the other hand, if the Ca content exceeds 0.20%, the amount of non-metallic inclusions increases and the high-temperature strength, stress corrosion cracking resistance, and toughness decrease, so the Ca content is 0.20% or less. The amount of Ca is preferably 0.15% or less.

Mg: 0.20% or less Mg is an arbitrary element, and the amount of Mg may be 0%.
Mg is an element that contributes to improvement in high-temperature strength and corrosion resistance with a small amount of addition. When the steel of this embodiment contains Mg, the amount of Mg is preferably 0.0005% or more, and more preferably 0.0010% or more from the viewpoint of obtaining the above effect more effectively.

  On the other hand, if the Mg content exceeds 0.20%, the strength, toughness, corrosion resistance, and weldability deteriorate, so the Mg content is 0.20% or less. The amount of Mg is preferably 0.15% or less.

Total of one or more of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re: 0.20% or less Lanthanoid elements other than Nd (ie, La, Ce, Pr, Pm, Sm, Eu) , Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu), Y, Sc, Ta, Hf, and Re are all arbitrary elements, and the total content of these elements is 0%. It may be.
Although lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re are expensive, they are elements that enhance the synergistic effect of the combined addition of Nd and B. When the steel of this embodiment contains one or more of these elements, the total content of these elements is preferably 0.001% or more, more preferably 0.005% or more. .

  On the other hand, when the total content of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re exceeds 0.20%, the amount of non-metallic inclusions increases, resulting in strength, toughness, corrosion resistance, and weldability. Therefore, the total content is set to 0.20% or less. The total content is preferably 0.15% or less.

The balance other than the above-described elements from the component composition of the steel of this embodiment is Fe and impurities.
An impurity here refers to 1 type, or 2 or more types of elements other than the element mentioned above. The contents of elements (impurities) other than the elements described above are each preferably limited to 0.010% or less, and preferably limited to 0.001% or less.

As for the component composition of the steel of this embodiment, the effective M amount Meff defined by the following formula (1) is 0.0001 to 0.250%.
Hereinafter, the effective M amount Meff will be described.

Effective M amount Meff = Nd + 13 · (B-11 · N / 14) −1.6 · Zr (1)
(In the formula (1), each element symbol indicates the content (% by mass) of each element.)

  The effective M amount Meff is an index that defines the quantitative relationship between Nd and B, which is indispensable for improving high-temperature strength and stress corrosion cracking resistance.

Expression (1) that defines the effective M amount Meff is a relational expression found by the present inventors from the viewpoint of securing excellent high-temperature strength and stress corrosion cracking resistance.
Formula (1) basically adds the amount of B that also functions effectively to the amount of Nd that functions effectively to ensure excellent high-temperature strength and stress corrosion cracking resistance, and has an excellent high temperature. It is a relational expression for subtracting the amount of Zr harmful to securing strength and stress corrosion cracking resistance.

In the steel of this embodiment, in order to ensure excellent high temperature strength and stress corrosion cracking resistance, N is reduced as much as possible to suppress the formation of nitrides.
However, when manufacturing steel industrially, a certain amount of N is inevitably mixed in the steel. If N mixed in the steel forms BN, the effect of B cannot be obtained. For this reason, it is necessary to secure B that is not bonded to N.

In the above formula (1) that defines the effective M amount Meff, the portion of “(B-11 · N / 14)” is not bound to N of the amount of B that functions effectively (that is, of the added B). B amount).
In the above formula (1), “(B-11 · N / 14)” (the amount of B not bonded to N) is multiplied by 13 to obtain “13 · (B-11 · N / 14)”. Weigh the amount of B that works effectively. Here, 13 times is the ratio of the atomic weight of Nd (≈144) to the atomic weight of B (≈11).
In the above formula (1), “13 · (B-11 · N / 14)” obtained above is added to the amount of Nd (“Nd + 13 · (B-11 · N / 14)”). Nd, like B, is an element that functions effectively to ensure excellent high-temperature strength and stress corrosion cracking resistance.

  In the above formula (1), in addition to “Nd + 13 · (B-11 · N / 14)”, the term “−1.6” which subtracts the amount of Zr harmful to securing excellent high temperature strength and stress corrosion cracking resistance.・ Zr ”exists.

The impurity element Zr forms nitrides and oxides and acts to reduce the synergistic effect of the combined addition of Nd and B.
In the formula (1), 1.6 (≈144 / 91), which is the ratio of the atomic weight of Nd (≈144) to the atomic weight of Zr (≈91), is multiplied by the Zr amount to obtain “1.6Zr”. , Zr weights the above-mentioned killing effect.
In the equation (1), the “1.6Zr” is subtracted from the “Nd + 13 · (B−11 · N / 14)”.

  As described above, according to the effective M amount Meff defined by the formula (1), the addition amount of Nd and B necessary for obtaining excellent high temperature strength and stress corrosion cracking resistance, and excellent high temperature strength and resistance. It is possible to quantify the amount of Zr that is harmful to securing the stress corrosion cracking property (specific examples will be described in detail in Examples).

If the effective M amount Meff is less than 0.0001%, it is difficult to obtain excellent high-temperature strength and stress corrosion cracking resistance. For this reason, the effective M amount Meff is set to 0.0001% or more. The effective M amount Meff is preferably 0.001% or more, more preferably 0.002% or more, and further preferably 0.010% or more.
The effective M amount Meff may be a negative value when the N amount or the Zr amount is large.

  On the other hand, if the effective M amount Meff exceeds 0.250%, the effect of improving the high temperature strength and stress corrosion cracking resistance due to the effective M amount Meff is saturated and the economy is reduced, and the strength, toughness, workability, and , Weldability decreases. For this reason, the effective M amount Meff is set to 0.250% or less. The effective M amount Meff is preferably 0.200% or less, more preferably 0.150% or less.

There is no restriction | limiting in particular in the metal structure of steel of this embodiment.
The metal structure of the steel of the present embodiment is preferably a coarse-grained metal structure from the viewpoint of further improving the high temperature strength (for example, high temperature creep strength of 700 ° C. to 750 ° C.).
Specifically, the steel of this embodiment preferably has an ASTM grain size number of 7 or less in the metal structure.
When the metal structure of the steel of this embodiment is a coarse grain structure having an ASTM grain size number of 7 or less, the grain boundary slip of the creep, the change of the metal structure due to the diffusion of elements through the grain boundary, and the sigma phase It is considered that the effect of suppressing the formation of precipitation sites is exhibited.
Therefore, it is preferable from the viewpoint of further improving the high temperature strength that the metal structure of the steel of this embodiment is a coarse grain structure having an ASTM grain size number of 7 or less.

In general steel, if the metal structure of the steel is a coarse metal structure, stress corrosion cracking tends to occur due to segregation of impurity elements at the grain boundaries.
However, in the steel of this embodiment, the segregation of impurity elements at the grain boundaries is reduced due to the high purity. For this reason, in the steel of this embodiment, even when it is a coarse-grained metal structure (for example, when the ASTM crystal grain size number of a metal structure is 7 or less), stress corrosion cracking is suppressed (that is, excellent Stress corrosion cracking resistance is maintained).

From the above viewpoint, the ASTM grain size number of the metal structure of the steel of this embodiment is preferably 7 or less, and more preferably 6 or less.
The lower limit of the ASTM grain size number of the metal structure is not particularly limited, but the lower limit of the ASTM grain size number of the metal structure is preferably 3 from the viewpoint of suppressing creep ductility and welding cracking.

As described above, the steel of this embodiment is excellent in high-temperature strength (particularly, creep rupture strength).
Although the specific range of the high temperature strength of the steel of this embodiment is not particularly limited, the steel of this embodiment preferably has a creep rupture strength at 700 ° C. for 10,000 hours of 140 MPa or more.

Here, 700 ° C. is a temperature higher than the actual use temperature.
Accordingly, the creep rupture strength at 700 ° C. and 10,000 hours of 140 MPa or more indicates that the high temperature characteristics are remarkably excellent.
Specifically, high temperature strength with a creep rupture strength of 140 MPa or more at 700 ° C. for 10,000 hours is 347H steel (18Cr-12Ni—Nb system) widely used in the world as a conventional 18Cr austenitic stainless steel. (See, for example, invention steels 1 to 20 and comparative steel 21 in Table 3 to be described later).

A creep rupture strength of less than 140 MPa can be easily achieved by extension of the prior art, but a creep rupture strength of 140 MPa or more is difficult to achieve by extension of the prior art.
In this regard, according to the steel of the present embodiment, carbide and creep can be achieved by optimizing the component composition, optimizing the effective M amount Meff by the Nd amount and B amount, increasing the purity by limiting the amount of impurity elements, etc. By the fine precipitation of the Laves phase precipitated therein, a creep rupture strength of 140 MPa or more (excellent high temperature strength) at 10,000 hours can be achieved at 700 ° C. higher than the actual use temperature.

There is no limitation in particular in the method of manufacturing the steel of this embodiment, The manufacturing method of well-known austenitic stainless steel can be employ | adopted suitably.
The steel of this embodiment may be a heat-treated steel plate or steel pipe.
The heating temperature in the heat treatment is preferably from 1050 to 1250 ° C, more preferably from 1150 to 1250 ° C, from the viewpoint of easily obtaining a coarse-grained structure and improving high-temperature strength (for example, creep rupture strength).
The mode of cooling after heating in the heat treatment is not particularly limited, and may be rapid cooling (for example, water cooling) or air cooling, but rapid cooling is preferable, and water cooling is more preferable.

The steel plate or steel pipe subjected to the heat treatment is prepared, for example, a steel plate or steel pipe having the component composition in the steel of the above-described embodiment, and the prepared steel plate or steel pipe is, for example, 1050 to 1250 ° C. (preferably 1150 ° C. to 1150 ° C. 1250 ° C.) and then cooled.
Any steel plate or steel pipe (steel plate or steel pipe before heat treatment) having the above component composition can be prepared according to a conventional method.
The steel pipe having the above component composition is, for example, cast a molten steel having the above-described component composition into a steel ingot or steel slab, and hot extrusion, hot rolling, hot forging to the obtained steel ingot or steel slab It can be prepared by performing at least one type of processing selected from the group consisting of cold drawing, cold rolling, cold forging, and cutting.

The steel of this embodiment has been described above.
There is no restriction | limiting in particular in the use of the steel of this embodiment, The steel of this embodiment is applicable to all the uses as which ensuring of high temperature strength and stress corrosion cracking resistance is requested | required.
The steel of this embodiment is a material steel suitable for heat-resistant pressure-resistant heat exchanger tubes or pipes such as boilers and chemical plants; heat-resistant forged products; heat-resistant steel bars; heat-resistant steel plates;
The steel of this embodiment is particularly a heat-resistant pressure-resistant heat exchanger pipe (for example, a heat-resistant pressure-resistant heat exchanger pipe having an outer diameter of 30 to 70 mm and a wall thickness of 2 to 15 mm) provided in the boiler, or a boiler pipe ( For example, it is particularly suitable as a raw material steel having an outer diameter of 125 to 850 mm and a wall thickness of 20 to 100 mm.

  Next, examples of the present invention will be described. The conditions in the examples are one example of conditions used for confirming the feasibility and effects of the present invention, and the present invention is based on this one example of conditions. It is not limited. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

In this example, 30 types of steels having component compositions shown in Tables 1 and 2 (continuation of Table 1) were melted.
In Tables 1 and 2, steels 1 to 20 are invention steels that are examples of the present invention (hereinafter also referred to as invention steels 1 to 20, respectively), and steels 21 to 30 are comparative steels that are comparative examples. (Hereinafter also referred to as comparative steels 21 to 30, respectively).

  The comparative steel 21 is a general-purpose 347H (18Cr-12Ni-Nb) steel, and is a standard material for comparing the prior art with the inventive steels 1 to 20.

When inventing inventive steels 1-20, high purity Fe obtained through blast furnace converter smelting and secondary refining by vacuum oxygen degassing is used as the Fe source, The high-purity alloying elements analyzed in 1) were used. Furthermore, before melting the inventive steels 1 to 20, the furnace for melting the inventive steels 1 to 20 was thoroughly washed, and special consideration was given so as not to introduce impurities.
In the production of invention steels 1 to 20, by the above special management, 6 impurities (specifically, Zr, Bi, Sn, Sb, Pb, and As), O amount, N amount, etc. are limited, Nd amount and B amount were controlled within appropriate ranges.

The high purity Fe source was also used when the comparative steels 23 to 30 were melted. However, in the melting of the comparative steels 23 to 30, the component composition was further adjusted as follows.
When the comparative steels 21, 23, 24, 27, and 29 were melted, at least one of the six impurities and O (oxygen) was intentionally added.
When the comparative steels 21, 24, and 26 were melted, N (nitrogen) was intentionally added.
When the comparative steels 21 to 23, 25, 27, and 28 were melted, at least one of B and Nd was not added.
When the comparative steel 21 was melted, the amount of Cu added was insufficient, and Mo, W, V, and Ti were not added.
When the comparative steel 30 was melted, the amount of W added was made insufficient.

-Explanation of Table 1 and Table 2-
-A numerical value shows content (mass%) of each element.
-The numerical value with an underline is a numerical value outside the range of the chemical composition in this embodiment.
-In each steel, the remainder except the element shown in Table 1 and Table 2 is Fe and impurities.
-Meff was calculated based on the above formula (1). Here, for a steel having a Zr content of less than 0.001% (indicated as “<0.001” in Table 2), Meff was calculated with the Zr content set to 0%.
Subtotal (X) indicates the total amount (mass%) of the six impurities (specifically, Zr, Bi, Sn, Sb, Pb, and As). Here, for elements with a content of less than 0.001% (indicated as “<0.001” in Table 2), a subtotal (X) was calculated with a content of 0%.

<Manufacture of test materials and heat treatment (1200 ° C.)>
Steels having the component compositions shown in Tables 1 and 2 were melted by vacuum melting and cast to obtain 50 kg of steel ingots.
The obtained steel ingot was hot forged to obtain a steel plate having a thickness of 15 mm.
By cutting the surface of the obtained 15 mm thick steel sheet, a steel sheet having a thickness of about 12 mm was obtained.
The obtained steel sheet having a thickness of about 12 mm was subjected to cold rolling at a cross-section reduction rate of about 30% to obtain a plate-shaped test material having a thickness of about 8 mm.
The test material was heated to 1200 ° C. and held for 15 minutes. After holding, the test material was subjected to heat treatment at 1200 ° C. by water cooling.

<Measurement of ASTM crystal grain size>
The ASTM crystal grain size of the test material after the heat treatment was measured according to ASTM E112. The measurement position of the ASTM crystal grain size was in the vicinity of the thickness center in the longitudinal section of the test material.
The results are shown in Table 3.

<Measurement of high temperature strength>
A creep rupture test piece having a diameter of 6 mm and a parallel part of 30 mm was cut out from the test material after the heat treatment with the longitudinal direction of the test material as the longitudinal direction. Using this creep rupture test piece, a creep rupture test was conducted at 700 ° C. for 10,000 hours or longer, and the creep rupture strength (MPa) at 700 ° C. for 10,000 hours was measured as the high temperature strength.
The results are shown in Table 3.

<Stress corrosion cracking test of base metal>
A test piece for corrosion having a width of 10 mm, a thickness of 4 mm and a length of 40 mm was cut out from the test material after the heat treatment. The cut specimen for corrosion is hereinafter referred to as “base material”.
The base material was subjected to a heat aging treatment at 650 ° C. for 10 hours.
A Strauss test (ASTM A262, Practice E: sensitization evaluation) was performed on the base material after the heat aging treatment to determine the presence or absence of cracks having a depth of 100 μm or more.
The results are shown in Table 3.

<Stress corrosion cracking test of welded HAZ (Heat Affected Zone) equivalent material>
A test piece for corrosion having a width of 10 mm, a thickness of 4 mm and a length of 40 mm was cut out from the test material after the heat treatment.
The cut out test piece was heated at 950 ° C. for 25 seconds using a greeble tester (in vacuum, energization heating). After heating, He was blown and cooled to obtain a welded HAZ equivalent material (welded heat affected zone equivalent material).
The obtained welded HAZ-equivalent material was subjected to a heat aging treatment and a Strauss test in the same manner as the stress corrosion cracking test of the base material, and the presence or absence of cracks having a depth of 100 μm or more was judged.
The results are shown in Table 3.

  As shown in Table 3, all of the metal structures of Invention Steels 1 to 20 and Comparative Steels 21 to 30 were coarse grain structures having ASTM grain size number 7 or less.

As shown in Table 3, the high temperature strengths of the inventive steels 1 to 20 were excellent strengths of 147 MPa or higher, and were about 1.5 times or higher than the high temperature strength of the comparative steel 21 (general-purpose 347H steel).
On the other hand, the high-temperature strength of the comparative steels 21 to 30 was a low strength of 137 MPa or less, which was inferior to the high-temperature strength of the inventive steels 1 to 20.

As shown in Table 3, in the inventive steels 1 to 20, no cracks having a depth of 100 μm or more were found in the inventive steel in both the base material and the welded HAZ equivalent material. From these results, it was demonstrated that the inventive steels 1 to 20 have excellent stress cracking resistance.
On the other hand, in the comparative steels 21 to 28, a crack having a depth of 100 μm or more was confirmed.

  More specifically, from the results of Comparative Steel 21 in which neither B nor Nd is added, and Comparative Steels 22, 23, 25, and 27 in which B is added but Nd is not added, the addition of Nd is It was proved to be effective in improving high temperature strength and stress corrosion cracking resistance.

  In addition, although Nd and B are added in combination, the amount of N is excessive, and the result of Comparative Steel 26 in which Meff is less than 0.0001% by mass indicates that the N amount is 0.0100% or less, and Meff is It has been demonstrated that 0.0001 to 0.250% is effective for improving the high-temperature strength and the stress corrosion cracking resistance.

Further, Meff is in the range of 0.0001 to 0.25%, but the amount of O is from the results of the comparative steel 24 in which the amount of O exceeds 0.0090% and the amount of N exceeds 0.0100%. It was demonstrated that the N content is 0.0090% or less and the N content is 0.0100% or less, which is effective for improving the high-temperature strength and the stress corrosion cracking resistance.
The reason why the high-temperature strength of the comparative steel 24 is low is presumed that Nd and B were consumed as oxides or nitrides, respectively, and fine precipitation strengthening did not occur.

Moreover, from the result of the comparative steel 28, it was proved that the B amount being 0.0010% or more is effective in improving the high temperature strength and the stress corrosion cracking resistance.
Moreover, from the result of the comparative steel 29, it was proved that the Zr amount being 0.002% or less is effective in improving the high temperature strength.
Moreover, from the result of the comparative steel 30, it was proved that the W amount is 2.00% or more effective in improving the high temperature strength.

<Relationship between grain size and stress corrosion cracking>
Inventive steels 1, 10, and 17 and comparative steels 21 and 23 were subjected to the following tests in order to investigate the relationship between the grain size of steel and stress corrosion cracking.

First, with respect to the test material subjected to the above-described heat treatment at 1200 ° C., the ASTM crystal grain size measurement, the base material stress corrosion cracking test, and the welded HAZ equivalent material stress corrosion cracking test were performed by the above-described methods. However, here, in the stress corrosion cracking test of the base material and the welded HAZ equivalent material, the depth of the crack was measured and the state of the crack was observed in detail.
The results are shown in Table 4.

Next, the test material before the above-described heat treatment at 1200 ° C. is heated to 1125 ° C., held at this temperature for 15 minutes, and then held and cooled with water, whereby the test material is heat treated at 1125 ° C. Was given.
For the test material subjected to heat treatment at 1125 ° C., ASTM grain size measurement, stress corrosion cracking test of base metal, and stress corrosion cracking of welded HAZ equivalent material in the same manner as the test material subjected to heat treatment at 1200 ° C. The test was conducted.
The results are shown in Table 4.

As shown in Table 4 and Table 3 above, in the inventive steels 1, 10, and 17 and the comparative steels 21 and 23, the metal structure of the test material heat-treated at 1200 ° C. is ASTM grain size number 7 or less. The coarse-grained structure.
On the other hand, as shown in Table 4, in the inventive steels 1, 10, and 17 and the comparative steels 21 and 23, the metal structure of the test material heat-treated at 1125 ° C. is a fine-grained structure having an ASTM grain size number of 8 or more. It became.

Moreover, as shown in Table 4, in the invention steels 1, 10, and 17, in any of the fine grain structure (ASTM crystal grain size number 8 or more) and the coarse grain structure (ASTM crystal grain size number 7 or less), Compared with the comparative steels 21 and 23, the stress corrosion cracking was sufficiently reduced.
In contrast to these invention steels, in comparative steels 21 and 23, cracking by the stress corrosion cracking test is possible in both cases of a fine grain structure (ASTM grain size number 8 or more) and a coarse grain structure (ASTM grain size number 7 or less). The depth of was 2 mm or more, and remarkable stress corrosion cracking occurred. In particular, many cracks of 3 mm or more occurred in the welded HAZ equivalent material.

  As described above, the inventive steels 1, 10 and 17 had significantly reduced stress corrosion cracking compared to the comparative steels 21 and 23.

The entire disclosure of Japanese Application 2015-114665 is incorporated herein by reference.
All documents, patent applications, and technical standards mentioned in this specification are to the same extent as if each individual document, patent application, and technical standard were specifically and individually described to be incorporated by reference, Incorporated herein by reference.

Claims (4)

Ingredient composition is mass%,
C: 0.05 to 0.13%,
Si: 0.10 to 1.00%,
Mn: 0.10 to 3.00%,
P: 0.040% or less,
S: 0.020% or less,
Cr: 17.00-19.00%,
Ni: 12.00 to 15.00%,
Cu: 2.00 to 4.00%,
Mo: 0.01-2.00%,
W: 2.00 to 5.00%
2Mo + W: 2.50 to 5.00%,
V: 0.01-0.40%,
Ti: 0.05 to 0.50%,
Nb: 0.15 to 0.70%
Al: 0.001 to 0.040%,
B: 0.0010 to 0.0100%,
N: 0.0010 to 0.0100%,
Nd: 0.001 to 0.20%
Zr: 0.002% or less,
Bi: 0.001% or less,
Sn: 0.010% or less,
Sb: 0.010% or less,
Pb: 0.001% or less,
As: 0.001% or less,
Zr + Bi + Sn + Sb + Pb + As: 0.020% or less,
O: 0.0090% or less,
Co: 0.80% or less,
Ca: 0.20% or less,
Mg: 0.20% or less,
One or more of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re: 0.20% or less in total, and
The balance: Fe and impurities,
Effective M amount Meff which is defined by the following equation (1) is Ri from 0.002 to 0.250% der,
An austenitic stainless steel having an ASTM grain size number of 9.6 or less in metal structure .
Effective M amount Meff = Nd + 13 · (B-11 · N / 14) −1.6 · Zr (1)
(In the formula (1), each element symbol indicates the content (% by mass) of each element.)   The component composition is one or more of Co: 0.01 to 0.80%, Ca: 0.0001 to 0.20%, and Mg: 0.0005 to 0.20% in mass%. The austenitic stainless steel according to claim 1, comprising:   The composition according to claim 1 or claim 2, wherein the component composition contains 0.001 to 0.20% in total of one or more of lanthanoid elements other than Nd, Y, Sc, Ta, Hf, and Re in mass%. Item 3. The austenitic stainless steel according to item 2.   The austenitic stainless steel according to any one of claims 1 to 3, wherein a creep rupture strength at 700 ° C for 10,000 hours is 140 MPa or more.